层状高导电性Ti3SiC2改性对锂离子电池低温性能的影响及其作用机理研究

Li ion batteries have been widely used in electronic digital products, electrical vehicles and so on. Temperature has an important role in the electrochemical performance of Li ion batteries. Li ion batteries have to face low-temperature environment in practical applications. However, the electrochemical performance of Li ion batteries at low-temperature is not satisfactory, especially below -20 C. At present, it is no simply way to completely solve this problem due to versatile and complicated factors. Usually, different measures are separately taken according to the corresponding cases. Based on analyses of various factors, it is pointed out that the charge transfer resistance, Rct, is the most important and significant one, influencing the electrochemical performance of Li ion batteries at low temperature. From this view of point, a strategy of Ti3SiC2 complex modification together with carbon is taken, focusing on a commonly used cathode material LiFePO4 and a new cathode material Li3V2(PO4)3, with a prerequisite of low coated carbon content. Ti3SiC2 is highly conductive with a layered structure. It gives a complex modification together with a small amount carbon coated on the cathode. The technique of the complex modification, microstructure, properties of the cathode powders and low-temperature performance of the Li ion batteries are investigated. The correlation between the complex modification and the low-temperature performance of the Li ion batteries and the mechanism are explored. The microstructure of the both cathode materials and conduction mode between cathode particles are changed through the complex modification. A new conducting mode form point-point mode to point-plane mode is constituted, resulting in a three-dimensional conduction network. As a result, the electrical conduction between particles is enhanced and Rct is remarkably reduced. Therefore, it is expected that the low-temperature performance of the Li ion batteries can be realized.

锂离子电池广泛应用于电子数码产品、电动汽车、军事及航空航天等领域，使用温度对其性能具有重要影响，在实际应用中，锂离子电池往往不得不面临低温下使用，然而现有锂离子电池低温性能差强人意（尤其在-20℃以下）。由于低温性能影响因素多而复杂，目前各国学者既无力也没有办法完全解决，只能针对不同因素，采取各个突破的策略。本项目分析众多影响因素，从最重要的电荷转移电阻Rct入手，针对常用的正极材料磷酸亚铁锂和新兴正极材料磷酸钒锂，在其碳包覆量较低的前提下，再采取层状高导电性Ti3SiC2改性的策略，研究液相法复合改性技术关键、材料微观结构特征与粉末性能以及低温电化学性能，探讨复合改性与低温电化学性能改善的内在关联与作用机理。经过复合表面改性，改变正极材料的微观结构，改变颗粒间的导电模式（由单纯点接触改为新增点面接触），构筑三维导电网络，以提高正极粉末电子电导率，显著降低Rct，可望明显改善低温性能。

锂离子电池因优异性能早已成为常温广泛应用储能设备，随着锂离子电池产品应用范围日渐扩大，对低温性能提出了更多更高要求，从机理上研究影响锂离子电池低温性能的因素并针对不同影响因素采取对应措施加以改善，意义重大。影响锂离子电池低温性能因素较多且复杂，电极材料Rct起着最为关键的作用。.以碳包覆量较低的磷酸亚铁锂和磷酸钒锂正极材料为对象，复合层状高导电性Ti3SiC2（TSC）或石墨烯，TSC或石墨烯将正极材料颗粒桥接在一起，改变了原来的颗粒之间单纯的点接触，增加了点-面接触模式，与碳共同形成三维导电网络，以期大幅改善低温性能。.研究改良的溶胶凝胶工艺制备了两种正极材料，引入TSC或石墨烯改性，重点对碳和TSC（或石墨烯）复合改性低温电化学性能进行研究，分析对低温电化学性能的影响及其作用机理。.不同温度下TSC改性均同时改善DLi+和电子电导，减小电极极化，获得了极低的Rct（5.6 Ω）。加入TSC后，10C下首次充放电比容量从37 mAh/g提高到114 mAh/g，10C下循环200次后，容量保留率为88%，明显高于未改性的材料。0℃时不同TSC含量的样品0.2C容量相差很小，1C时容量相差明显增大，说明低温下TSC改性效果随着倍率升高更加显著。-20℃下，TSC改性样品循环50次后容量没有明显衰减，T4样品仍然保有容量80 mAh/g，而T0的容量几乎为零，说明TSC改性可以显著改善电极低温下的结构稳定性和循环性能。随着温度降低，所有样品DLi+逐渐减小，但改性样品的DLi+均高于未改性的样品。.随着降低温度和升高倍率，石墨烯改性效果更加明显，充放电平台间的电压差变小，充放电平台延长。-20℃时石墨烯加入量为0wt%、3wt%、5wt%、7wt%的样品在1C倍率下的首次充放电比容量分别为82、91、98和89mAh/g。低温下石墨烯含量5wt%的电极材料表现出了最好的倍率循环性能和电化学可逆性。0℃、1C下循环200次后各试样容量剩余分别为83%、92%、94%和89%。-20℃、1C下循环200次后G5试样比G0试样放电比容量高31mAh/g。低温下G5表现出最小的氧化还原峰电势差、最小的Rct和最高的DLi+，即在低温下具有最好的电化学动力学。